This invention generally relates to the reproduction of selected images by the electrostatic attraction of development fluid such as liquid ink onto a receiving medium. More particularly, the invention relates to an improved article of manufacture, sometimes referred to herein as a gravure roll, used for transferring predetermined quantities of development fluid from a source to the receiving medium.
Fluid development of electrostatic images is not new, an early system embodying such a concept being disclosed in U.S. Pat. No. 3,048,043 issued to R. W. Gundlach on Apr. 2, 1963 and assigned to the instant assignee. Though specific prior art fluid development systems vary, a typical apparatus may include a rotatable drum having a photoconducting surface and an electrically conductive substrate. In general, latent electrostatic images are formed on the photoconducting surface by uniformly charging the surface thereof, as by a corona discharge device followed by exposure to light in the desired image pattern and development by liquid developers. Development may be carried out on the photoconducting surface itself or on other receiving means. The receiving means may be an interposer member or any other suitable means moved between a roller and the rotatable drum, thereby contacting a portion of the photoconducting surface at all times. The developed image pattern may be transferred from the photoconducting surface or the receiving means to a suitable support surface, such as paper, metal, polymer film and the like.
In fluid development systems having a receiving means, a portion of the photoconducting surface, not then rotated into contact with the receiving means such as the transparent interposer, is uniformly electrostatically charged by any suitable means. The charged portion of the photoconducting surface is then subjected to a light and shadow representation of the selected image. Accordingly, the original charge applied to the photoconducting surface, upon receipt of the light and shadow representation, assumes a pattern corresponding to the image to be developed, said pattern subsequently being capable of development on the receiving means (interposer).
Electrostatic imaging systems of the prior art further include a source of development fluid, and patterned means adapted to receive development fluid from said source. The particular patterns formed in the patterned means may vary, though, as explained in U.S. Pat. No. 3,801,315 issued to Robert W. Gundlach, et al. on Apr. 2, 1974 and assigned to the instant assignee, one specially well-suited pattern is a triangular-helix. As the name implies, this pattern comprises three parallel volute grooves disposed in the surface of a cylinder. To many of those skilled in the art, such a cylinder is sometimes referred to as a trihelicoid gravure roll, and that terminology is followed herein.
Upon receipt of development fluid, a doctor blade is conventionally applied to the gravure roll in an effort to remove excess fluid which would otherwise interfere with the accurate reproduction of the selected image. The doctored, fluid-carrying gravure roll is then typically rotated into facing relationship with the photoconducting surface of the rotatable drum, or in the case where there is an interposing surface at a point where the interposer is in contact with the photoconducting surface. Development fluid is then electrostatically attracted to the portion of the interposer overlying the charged portion of the photoconducting surface or to the charged portion of the photoconducting surface itself.
After development, the photoconducting surface or the interposer is moved into contact with a copying medium such as a sheet of copy paper. A pressure roller, adapted to urge the copy paper against the interposer or photoconducting surface, facilitates passage of development fluid from the interposer or photoconducting surface to the paper. Since the development fluid on the interposer or photoconducting surface is typically in the form of the selected image, that image is transferred to the paper. After development, the photoconducting surface is discharged through the conductive substrate, thereby eliminating any residual electrostatic charges remaining on the photoconducting surface. At substantially the same time, excess development fluid on the interposer or the photoconducting surface is removed by means such as a doctor blade, thereby readying the apparatus for the subsequent development of other selected images.
Though fluid development systems of the type described have been used to reproduce selected images, they are subject to numerous drawbacks and deficiencies. For example, it is difficult to transfer accurate quantities of development fluid to the gravure roll for subsequent deposition onto the photoconducting surface or the receiving means such as a transparent interposer. As a result, insufficient amounts of fluid are sometimes attracted to the surface to be developed, thereby preventing portions of the selected image from being fully developed. Alternatively, too much fluid is often attracted to the surface to be developed, causing an undesirable blotching effect. Accordingly, it is a primary object of this invention to provide means for assuring that the proper amount of development fluid is passed from the gravure roll.
Another problem frequently associated with fluid development systems of the type described relates to the doctoring of excess ink from the gravure. More particularly, the doctor blade frequently used in conventional fluid development systems must be applied with sufficient force to insure removal of all excess fluid from the gravure roll, particularly the high points or lands thereof. If such exesss fluid is not removed, blotching will inevitably occur.
When the doctor blade is forcefully applied to the gravure roll, however, it is often urged into the trihelicoid grooves, thereby withdrawing development fluid therefrom. As a result, there may be too little fluid remaining in the grooves for proper attraction onto the receiving medium. In addition, the withhdrawal of the fluid from the grooves in the gravure roll alters the distance between the receiving medium and the development fluid. If this distance becomes too great, the electrostatic forces may be insufficient to properly draw the fluid to the recording medium. On the other hand, if the distance is too small, development fluid could be attracted to areas on the recording medium where fluid should not be deposited. Accordingly, it is a further object of the invention to provide an improved fluid development system wherein the distance between the receiving medium and the development fluid carried by the gravure roll is not appreciably changed upon doctoring the gravure roll.